Most printed circuit boards (PCBs) manufactured through 1985 had electroplated tin/lead on the top of the circuitry. In these typical PCBs of the time, the PCB was composed of a copper conductor pattern on an insulating support, with the tin/lead solder applied onto the copper substrate, typically by electroplating. The tin/led solder film was typically on the order of 0.0003 to 0.0004 inches thick, which thickness might vary considerably over the board and from board to board. After the solder film was applied onto the copper, a thin film of copper-tin alloy, called by the industry and "intermetallic layer", forms between the copper and the solder, typically about 0.000002 to 0.000004 inches thick. This copper-tin alloy film increases in thickness with time. This tin/lead coating served at least two purposes: 1) to facilitate the manufacturing process by serving as an etch resist; and 2) to preserve the solderability of the PCB.
Later, as the dimensions of the circuitry became smaller, the tin/lead coating became problematic. During the process of attaching the component(s) the tin/lead coating melted, ran, and electrically shorted the circuitry. To address this problem, a new type of PCB was developed which involved stripping the electroplated tin/lead from the board, and applying a solder mask, hence the term solder-mask-over-bare-copper (SMOBC) board. While the SMOBC method of PCB manufacturing solved the problems posed by the tin/lead coating in PCB manufacturing, the approach created a different, and very serious, problem. First, the waste generated by the stripping process posed a significant health and environmental hazard, particularly due to the presence of lead. In response, most PCB manufacturers today use a metal coating that is essentially pure tin, rather than a tin/lead alloy. The pure tin solder also had the advantage that it allowed an even thinner metal layer, typically about 0.0002 inches thick.
Several different approaches to accomplishing stripping of the tin and tin/lead solders have been described. In general, most of these process involve a source of ferric ions in an amount sufficient to dissolve the tin-copper alloy created at the solder-board interface. The stripper solutions are also generally of a very acidic pH, due to the addition of, for example, nitric acid and/or sulfamic acid. Exemplary stripper solutions are described in U.S. Pat. Nos. 5,512,201; 5,244,539; 4,713,144; and 4,687,545.
Despite the developments to overcome the basic manufacturing problems, serious obstacles to cost-effective PCB production remain. First, the waste generated from stripping the tin/lead solder, and even the pure tin replacement, pose significant environmental and health hazards. As mentioned above, the strip waste from the tin/lead solder stripping process contains hazardous amounts of lead. Even the waste from stripping the pure tin solder must be treated as a hazardous waste, since the stripping inevitably results in removal of copper from the board when the solder covering it is stripped from the board. Thus, in general, the spent tin, or tin/lead, stripper typically contains about 150 to 200 gm/l of dissolved, or dispersed metal(s) (e.g., tin or tin/lead), with small amounts (about 100 to 2000 ppm (or about 0.01 to 2.0 gm/l)) of copper present in solution. The tin is present mostly as stannic oxide, the lead as plumbous nitrate, and the copper as cupric nitrate. Disposal of such wastes is extremely expensive, and effective, low cost methods for treating such waste to render it more environmentally safe are sorely lacking.
In addition to the costs associated with disposal of the spent stripper solution, disposing of spent stripper solution also represents potentially millions of dollars in lost opportunity costs. Specifically, the large amounts of tin in the spent stripper solution are very valuable, adding to the overall operation costs. For example, at this writing, the typical cost of a commercial proprietary tin or tin/lead stripper solution costs about $6 to $7 per gallon, the cost of disposal of the spent stripper solution is about $3 to $5 per gallon, and the value of the tin in the spent stripper solution, since the stripper capacity is typically about 1 to 1.5 lbs per gallon of tin, is about $3 to $5 gallon.
Despite the fact that these estimates of costs and lost value would seem sufficient to motivate tin recovery from spent stripper solutions, there are few cost-effective means for successfully removing the tin from these wastes. For example, one conventional approach for cleaning up the solution and recovering the metal values is a specialized form of electroplating called electrowinning. However, when simple electroplating/electrowinning technology is applied, because the tin is simply dispersed, and not in true solution, the tin simply does not plate out of the spent stripper solution effectively.
There is thus a need in the field for methods and compositions to recover tin from waste solutions in a cost-effective manner and that further provides an environmentally safer treated waste solution. The present invention addresses these problems.